This invention relates to vacuum pumps and, more particularly, to pumps employing molecular drag mode of operation, preferably in conjunction with a regenerative mode of operation.
Vacuum pumps and/or compressors are known which operate in a regenerative mode and in which a rotor spins at high speed, for example ten thousand revolutions/min (10,000 rpm), within a stator body and in which the rotor has a series of blades positioned in an annular array either on a peripheral edge of the rotor or alternatively on a side of the rotor at its periphery, and the stator has an annular channel within which the blades rotate having a cross sectional area greater than that of the individual blades except for a small part of the channel known as a xe2x80x9cstripperxe2x80x9d which has a reduced cross section providing a close clearance for the blades.
In use of such a pump, gas to be pumped enters the annular channel via an inlet positioned adjacent one end of the stripper and the gas is urged by means of the blades on the rotating rotor along the channel until it strikes the other end of the stripper and the gas is then urged through an outlet situated on that other end of the stripper. It is known that pumps/compressors employing such a mode of operation can provide a high compression ratio at relatively low flow rates. However, these pumps are best suited to pumping in continuum flow conditions and with such pumps it can be difficult to obtain a sufficiently high ultimate vacuum and pumping speed without resort to the use of an additional vacuum pump in tandem which is suited to transitional and/or molecular flow.
In our earlier European Patent Application No. 0 805 275 A, we described a vacuum pump in which a substantially higher compression could be obtained through the use of a multi-stage pumping action associated with the rotor in particular. In our earlier European Application, there was disclosed a vacuum pump of the regenerative type comprising a rotor and a stator body in which the rotor was adapted for rotation and in which the rotor had a series of blades positioned in an annular array on a side of the rotor and the stator had an annular channel within which the blades could rotate having a cross-sectional area greater than that of the individual blades except for a small part of the channel which had a reduced cross-section providing a close clearance for the blades and wherein the rotor had at least two series of blades positioned in concentric annular arrays on a side of the rotor and the stator had a corresponding number of channels within which the blades of the arrays could rotate and means were provided to link the channels to form a continuous passageway through which gas being evacuated by the pump could pass.
It was further disclosed that the pumps of the earlier European application may be employed:
i) as individual vacuum pumps in their own right,
ii) in conjunction with other vacuum pumps such as turbo molecular pumps or molecular drag pumps,
iii) as a component part of larger hybrid vacuum pumps also comprising stages of different type, for example molecular drag stages.
In particular, it was disclosed that hybrid pumps comprising a regenerative stage according to the earlier European Application together with a type of molecular drag stage, for example are known as a xe2x80x9cHolweckxe2x80x9d stage, were particularly beneficial.
In a Holweck stage, there is provided alternate stationary and rotating concentric hollow cylinders with a threaded upstanding flange to form a helical structure substantially extending across the gap between adjacent cylinders, the flange being attached either to a surface of a rotating or of a stationary cylinder.
It was found that such pumps, especially the regenerative/molecular drag compound pumps, were generally able to provide a higher ultimate vacuum together with a relatively higher compression ratio than that available with comparable vacuum pumps.
Nevertheless, the pumps of our prior European application can in some circumstances suffer from the disadvantage that a sufficiently higher pumping capacity cannot always be attained.
This invention is concerned with a modified design of vacuum pump in which this and other disadvantages are removed.
In accordance with the invention, there is provided a vacuum pump for pumping gas from a pump inlet to a pump outlet, comprising a rotor and a stator body in which the rotor is adapted for rotation and including at least two molecular drag stages each comprising adjacent stationary and rotating Holweck cylinders attached to the stator body and the rotor respectively and with a threaded upstanding helical flange positioned therebetween which is attached either to the stationary or to the rotating cylinder wherein the molecular drag stage closest to the pump inlet has the threaded flange on its rotating cylinder and the subsequent molecular drag stage or stages has the threaded flange on the stationary cylinder
A central feature of the invention is the presence of a threaded flange on the rotating Holweck cylinder of the first Holweck stage, ie the Holweck stage closest to the pump inlet. It is this featurexe2x80x94coupled with the reverse positioning of the flange in the subsequent Holweck stagexe2x80x94which allows the pump overall to exhibit generally superior properties with particular reference to the high pumping capacity and compression and low power consumption at higher (inlet) pressures.
In addition, it is extremely advantageous for the Holweck stages to be arranged in a radial configuration, in particular adapted so that the gas being pumped flows radially outwards from the first Holweck cylinder to a subsequent, radially arrayed Holweck cylinder or cylinders. The additional benefits of such pumps include:
i) a lower power consumption in continuum flow than with axially arranged Holweck cylinders of comparable outside diameter
ii) an ability to assemble and dis-assemble the rotor from the pump body
iii) compactness of the pump overall
The pumps of the inventions preferably also include a regenerative stage towards the outlet end of the pump such that gas being pumped enters the regenerative stage following its exhaustion for the molecular drag stages. Preferably, the regenerative stage comprises a series of blades positioned in an annular array on one or both faces of the rotor or on an edge of the rotor.
In preferred embodiments, the rotor has at least two series of blades positioned in concentric annular arrays on a face of the rotor and the stator has a corresponding number of channels in which the arrays of blade can rotate.
In such embodiments, the blades advantageously extend in a substantially axial direction. The rotor is preferably shaped such that the side on which the arrays of blades are positioned presents a substantially flat surface for receiving the arrays; usually, the flat surface will be radially orientated relative to the main axis of the rotor. Generally, the flat surface between the arrays will cooperate with corresponding annular flat surfaces on the stator to provide a face seal between the arrays.
If appropriate, the invention also incorporates the possibility of there being at least two arrays of blades on each side of the rotor, each side preferably presenting a substantially flat surface for receiving the arrays.
In preferred embodiments, the rotor has at least five or six arrays on one or both sides thereof.
The individual blades of each array will generally be arranged radially in relation to the rotor. Each blade may be substantially flat or, at least in part, may be arcuate with the concave side pointing in the direction of travel of the rotor; the latter is preferred to assist in pumping efficiency.
It is preferred for the blade edges which co-operate with the stripper to have a flat surface rather than pointed or radiused ends to improve the xe2x80x9csealingxe2x80x9d between the blades and the stripper.
Typically, each array may comprise at least about ten, preferably at least fifty blades. Generally, there may usefully be up to about one hundred and fifty blades in each array. Preferably the cross-sectional area of the main part of the channel is from three to six times that of the radial cross-section of the blade.
Having more than one series of blades in annular arrays on the surface of a rotor in accordance with the invention affords various advantages and opportunities. Firstly the arrangement of the blades and corresponding channels in a series of concentric arrays relative to the pump shaft can provide an inherent volumetric compression ratio if a flow of gas being evacuated is caused to occur from the outermost array to the innermost array to exhaust towards the centre of the pump. This effect is increased if the cross-sectional area of the individual channels is decreased gradually from the outermost to the innermost channel. For example, in a pump having six such arrays, the cross-sectional area of the innermost channel may be of the order of one-sixth to one-half of that of the outermost channel.
Secondly, the concentric arrays of blades/channels allows for a shorter pump overall in the axial direction than one with a multi-stage axial array of blades.
Thirdly, the axial load can be reduced, in particular if the flow of gas is arrayed from the outside to the inside channel, because of the highest pressure forces in such an arrangement are at the centre of the pump and act over a smaller area.
Fourthly, use of a particularly preferred feature in which each array of blades is mounted on a raised ring present on the surface of the rotor with the corresponding stator channels being present about the blades to allow rotation of the blades therethrough but with a relatively close tolerance between the stator and the curved surfaces of the raised ring provides the opportunity of radial sealing between the rotor and the stator.
In the molecular drag stage it has been found to be particularly useful to arrange the Holweck cylinders axially with the spinning cylinder(s) being mounted on the same shaft as the spinning rotor of the regenerative stage.
In conjunction with the regenerative pump stage of the invention in which the rotor blades will generally depend axially from the rotor, a corresponding axial arrangement of the Holweck cylinders is preferred. In combination with the regenerative blades on the rotor, this forms a pump that has no radially interleaving stator sections, thereby allowing ready assembly and dis-assembly of the pump.
In such respects, it is preferred for one pump stage to be on one side of the rotor and the other stage to be on the opposite side of the rotor. This feature affords the possibility of a smaller, lighter pump overall.
The Holweck stage will in particular generally be at the inlet (high vacuum or low pressure) end of the pump and such an axial arrangement of the Holweck cylinders has been found to provide a natural inlet for the pump as a whole by causing gas to enter through the innermost cylinder. It can advantageously be arranged for gas flow in the Holweck stages to be from the centre outwards and in the regenerative stages to be from the outer periphery inwards, thereby leading to a balanced, efficient pump overall.
In combined regenerative/Holweck pumps, the general design lends itself advantageously to a single piece rotor which can usefully be made of a light metal or alloy, for example aluminium.
In preferred embodiments, the rotating Holweck cylinder of the first molecular drag stage has a threaded flange of greater radial flange depth overall in comparison with that of the subsequent Holweck stage or stages. This allows for a greater pumping capacity generally. Furthermore the threaded flange of the first Holweck stage advantageously may possess a variable thread pitch and/or thread channel depth. The presence of one, preferably both, of these generally allows for low power consumption in operating the pump, particularly at high (inlet) pressure operation, coupled with suitable performance at low (inlet) pressures. It is the combination of having a rotatable Holweck flange and the deep thread or channel depth which allows for lower power consumption in operating the pump, especially at high inlet pressures, coupled with good performance at low inlet pressures.
In these preferred embodiments, the pitch is advantageously varied such that the pitch gradually decreases in a direction away from the pump inlet and the thread depth is also advantageously varied such that the depth gradually decreases in a direction away from the pump inlet such that they offer a high pumping capacity at the inlet to the stage.
Pumps of the invention having in particular the Holweck cylinders arranged in a radial direction, with gas being pumped during operation of the pump being urged from a centrally positioned inlet in a generally radially outward direction through the
Holweck stages and with the regenerative stages being positioned axially beneath the Holweck stages, for example with the blades arranged on a face of the rotor in a direction pointing generally away from the Holweck stages such that, in particular the regenerative stages are similarly radially arranged, have the advantage that pumps of the invention may be of overall compact design and, in addition, be made available in different constructional modules.
For example, a standard platform module may include a simple rotor disc on a lower face of which depend the blades of the regenerative stage and on an upper face of which is a single axially depending Holweck stage comprising a stationary, flanged Holweck cylinder and a rotating, non-flanged Holweck cylinder.
A second module may additionally comprise an additional Holweck stage with a further rotatable, non-flanged cylinder; subsequent module may additionally comprise further rotatable non-flanged Holweck cylinders.
A final module may comprise the complete pump according to the invention including a further Holweck stage comprising a rotatable, flanged cylinder, preferably with a variable pitch and/or flange depth, nearest the pump inlet.